On 1 June 2003, Belgium became the second country in the world to legalize same-sex marriage, after the Netherlands. "Statutory cohabitation", open to any two cohabiting persons, is also possible since 1 January 2000.

In the late 1990s, gay rights organizations in Belgium lobbied for the legalization of same-sex marriage. Belgian civil law did not explicitly require that two people be of opposite gender to be able to marry, as this was considered self-evident. Private member's bills in the 1990s by Vlaams Blok senators to add this as an explicit requirement were never considered.[1][2]

In 1995, a bill was introduced in Parliament to provide for a legal framework of "cohabitation agreements". It was mostly intended as a response to the lowering marriage rates, rather than giving rights to same-sex couples. In 1998, the bill was changed to "statutory cohabitation" (Dutch: wettelijke samenwoning; French: cohabitation légale) and finally voted on. The Chamber of Representatives approved it by a 98–10 vote with 32 abstentions and the Senate by a 39–8 vote with 19 abstentions. The Act of 23 November 1998 gives limited rights to registered same-sex and opposite-sex couples by inserting a Title V-bis on statutory cohabitation in the Belgian Civil Code and by amending certain provisions of the Belgian Civil Code and the Belgian Judicial Code. However, being a couple is not a requirement to make a declaration of statutory cohabitation; relatives can do so too. The law was legally published on 12 January 1999 but not yet in effect.

The election programmes of the SP (Flemish social democrats), Agalev (Flemish Greens) and VLD (Flemish liberals) for the 13 June 1999 elections included the aim to legalise same-sex marriage. The new Verhofstadt I Government was formed, which was notably made up of a coalition of liberal, socialist and green parties and excluded the long-dominant Christian democrats, who lost the elections due to the Dioxin Affair. The coalition agreement included "implementing a full legal partnership scheme" as well as "immediately making the Act of 23 November 1998 enter into force", which had not been done yet.[3] A royal order signed on 14 December and published on 23 December 1999 made the Act of statutory cohabitation go into effect on 1 January 2000.[4]

During 1999, the PS (French-speaking social democrats) and Ecolo (French-speaking Greens) also announced they agree to legalise same-sex marriage. At that point, the only remaining party in government that opposed same-sex marriage was the French-speaking liberal PRL (later merged into MR), mainly because it was opposed to adoption rights for same-sex couples. PRL agreed not to block same-sex marriage if adoption rights are excluded. As the first same-sex marriage in the Netherlands was performed on 1 April 2001, the Belgian government, mostly under the lead of Minister Magda Aelvoet (Agalev), began considering it as well.[5][6] In June, the cabinet of ministers formally approved opening marriage to same-sex couples.[7] In September, then-opposition party CD&V (Flemish Christian democrats) agreed to support the bill. On 30 November 2001 however, the Council of State gave a negative legal opinion on the bill, saying that "marriage is defined as the union of a man and a woman".[8] LGBT organisations and government ministers criticised the opinion and said they would proceed with the legislation.[9]

(Two persons of different sex or of the same sex may contract marriage.)

In October 2004, the Arbitration Court, now known as the Constitutional Court, rejected an attempt by opponents of same-sex marriage to have the law declared unconstitutional.[13] The main argument held that treating fundamentally different situations the same way, violates the equality principle of the Constitution.

Originally, Belgium allowed the marriages of foreign same-sex couples only if their country of origin also allowed these unions. Legislation enacted in October 2004 however, permits any couple to marry in Belgium if at least one of the spouses has lived in the country for a minimum of three months.

The same-sex marriage law did not permit adoption by same-sex partners, and as birth within a same-sex marriage did not imply affiliation, the same-sex spouse of the biological parent had no way to become the legal parent. A proposal to permit adoption was approved 77–62 (with 7 abstentions) by the Chamber of Representatives on 1 December 2005,[14][15] and 34–33 (with 2 abstentions) by the Senate on 20 April 2006.[16] It received royal assent on 18 May 2006 and went into force on 30 June 2006.[17]

A legal inequality compared to heterosexual couples still existed with regards to children: the husband of the biological mother is automatically legally recognised as the father (by article 135 of the Civil Code), but this was not the case in a same-sex couple for the wife of the mother. To be recognised as the co-mother, she had to complete an adoption procedure. This accounted for the large majority of adoption cases in Belgium. The Di Rupo Government promised to fix this, and in 2014, as the Netherlands recently passed similar legislation, LGBT organisations pressured the government about their promise. Subsequently legislators worked to agree on a solution.[18] A bill to this end was approved by the Senate on 3 April 2014 on a 48–2 vote (with one abstention), and by the Chamber of Representatives on 23 April on a 114–10 vote (with one abstention). The bill received royal assent on 5 May and went into effect on 1 January 2015.

Since this change, female same-sex couples are treated equally to heterosexual couples: the co-mother married to the mother is automatically recognised as parent, and an unmarried partner can formally recognise the child at the civil registry. An equivalent solution for male same-sex couples has not been agreed upon, due to the controversy surrounding surrogacy.

Mayor of Liège, Willy Demeyer, officiating the wedding of a gay couple

According to the Belgian Official Journal, approximately 300 same-sex couples were married between June 2003 and April 2004 (245 in 2003 and 55 in 2004). This constituted 1.2 percent of the total number of marriages in Belgium during that period. Two thirds of the couples were male and one third female. On 22 July 2005, the Belgian government announced that a total of 2,442 same-sex marriages had taken place in the country since the extension of marriage rights two years earlier.[19] The following table summarizes the number people married in same-sex marriages[20]